The present invention relates to the preparation of optical elements by cold working of formable (i.e. ductile) crystals. More specifically, the present invention relates to the preparation of elements for infrared applications, using processes such as cold forging, embossing, or stamping, which are carried out in a narrow temperature range, using a sacrificial split die. The present invention also relates to the use of such elements, and in particular of thin waveguides, as the bases for a system that can be applied for the diagnosis of tissues or biological fluids, in medicine. The same diagnostic system could be used in cosmetics for skin analysis or for the measurement of the diffusion of medical drugs or cosmetics into the skin.
Infrared (IR) radiation covers a wide spectral range which can be divided into three ranges: NIR=near IR (λ=1 μm-3 μm); MIR=middle IR (e.g. λ=3 μm-30 μm) and FIR=far infrared (λ=30 μm-100 μm). There are several scientific, industrial, medical and military applications that make use of infrared radiation and in particular of MIR radiation. Among these applications are the following:    IR detection: A warm body at a temperature T emits radiation (“black body” radiation”) whose intensity I is given by the formula: I=εσT4 where ε & σ are constants. For bodies near room temperature most of this emission is in the MIR range, and suitable IR detectors can measure 1. The measurement of this radiation serves for military applications such as detection of personnel or vehicles in total darkness. It also serves scientific and medical applications such as non-contact determination of the surface temperature T.    Thermal Imaging: The distribution of intensities emitted from a large area can be determined by a large array of IR detectors and displayed on a monitor. This has been used mostly for military applications (e.g. FLIR), because the arrays used were very expensive. A new generation of inexpensive arrays will probably be used for medical and industrial applications.    IR Lasers: Numerous lasers emit IR radiation, including solid-state lasers, such as Nd: YAG (λ=1.06 μm) or Er: YAG (λ=2.96 μm) and gas lasers, such as CO2 (λ=10.6 μm) or CO (λ=5.3 μm). MIR lasers have been used for military applications, such as target designators, for industrial applications, such as cutting, heating and welding of materials, and for medical applications, such as laser surgery and laser therapy.    IR Spectroscopy: There are many organic and inorganic molecules that have characteristic absorption in the MIR (called “finger print”). This absorption serves as an important tool for material analysis. The absorption of a sample can be measured using standard (grating) spectrometers or Fourier Transform Infrared (FTIR) spectrometers, or tunable laser spectrometers. The absorption spectrum is used for determining the chemical composition of the sample. This has been widely used in industry and science. IR spectroscopy is potentially useful in biology and medicine, for IR clinical chemistry, and for IR pathology.
Some authors use the term “optical” only for the visible spectral range. Herein, all the elements that are transparent in the infrared and are used in IR system are referred to as “infrared optical elements”.
Optical systems that are designed for visible light, are based on standard optical elements. Most of these optical elements make use of standard materials such as silica (SiO2) based glasses or transparent plastics. All these materials are completely opaque in the MIR and other families of materials have to be used in infrared systems. These include crystals such as NaCl, KBr, BaF2, MgF2, Ge, Se, ZnSe, ZnS, or glasses such as As2S3, As2Se3, ZrF etc. Each of these materials is transparent in a different range in the infrared. See J. A. Savage, Infrared Optical Materials and Their Antireflection Coatings, Adam Hilger, Bristol and Boston, 1985.
Various optical elements are prepared from the IR transparent materials. These optical elements include small and large windows, convex or concave lenses, various prisms and miniature (refractive) optical elements. Such IR elements are generally made by techniques that have been previously used for silica based glasses: a piece of material is cut to the desirable dimensions, it is ground to the rough shape and finally it is polished to generate an exact shape with high surface quality. Elements of this type are referred to herein as “bulk” elements. The fabrication processes have been adapted for each of the infrared transmitting materials mentioned above. In many of these cases the process is much more complicated than for silica, because many of the IR materials are difficult to handle: they are brittle, water soluble or toxic. In all cases the process of making IR optical elements is complicated, time consuming and expensive, especially when trying to make large optical elements.
Arrays of microlenses (lenslet arrays) that are purely refractive have been fabricated on the surface of silica-based glasses, using various techniques. In the case of glasses, a technique called reflow has been used. For glass and other materials one can use direct fabrication methods such as diamond turning, or etching through a photoresist mask. Circular, hexagonal, square and cylindrical arrays have been formed, with individual lens size of the order of tens of microns. Similar techniques have been used for infrared transmitting glasses and crystals, such as Ge, Si and ZnSe. Manufacturers of such optical elements include MEM Optical Inc. of Huntsville, Al. and Adaptive Optics Associates of Cambridge, Mass.
There are two new families of surface-relief micro-optical elements: continuous relief optical elements and binary optical elements. Such diffractive optics elements (DOE) are called planar, because their relief amplitude is normally less than 10 μm. These optical elements are designed to convert an incoming light beam into a predetermined output beam. The output beam can be focused into a point, or shaped like a cross, or a ring or any other light pattern. Diffractive optical elements are used as lenses, beam splitters, diffusers etc. Towards this goal, sets of specially designed microscopic patterns are generated on the surfaces of standard, silica based, optical elements. The techniques for generating such patterns are normally based on photolithography, a method that had been borrowed from the semiconductor industry. These techniques involve the generation of a photoresist pattern on the optical element and etching the surface of the element through the pattern by chemical etching or dry etching. The diffractive optical elements can also be made on surfaces using diamond turning or laser ablation. See, for example, J. Turunen and F. Wyrowski, Diffractive Optics, Akademie Verlag, 1997. Some of these processes have been adapted for the fabrication of diffractive optics IR elements, notably by the Coherent Auburn Group of Auburn Calif., but the processes are time consuming and rather expensive.
Hybrid optical elements employ at least two of the optical technologies mentioned above. For example optical elements are made with refractive and with diffractive surfaces. This combination is used to realize some specialized optical functions. One such hybrid optical element is a traditional plano-convex lens, where a diffractive element is fabricated on the plane surface.
In the visible range use has been made of highly transparent plastics such as PMMA (i.e. Lucite or Plexiglas). One of the main advantages of these plastics is that they are softer than glass and have lower melting points. It is therefore possible to mold them or to deform them at relatively low temperatures to a desirable shape. It has been found that it is possible to prepare in this fashion molded optical elements of high optical quality and very low cost. A mold, in which the cavity is formed by two or more components held together, is called a split cavity mold. In this case, after molding, the components of the mold are separated from the molded piece at the end of the molding process. This permits to form undercuts in the molded piece. These components are re-used for molding a new batch of optical elements.
These methods lend themselves easily to mass production. Such molding procedures can easily replicate a “master” and they serve in a plethora of domestic, industrial, scientific and military applications. Replication techniques have been also used for the fabrication of refractive microlens arrays from PMMA or polystyrene, for applications in the visible.
Diffractive or binary optical elements and purely refractive micro-optical elements have been mass-produced in large quantities by replication techniques. This involves the transfer of surface relief profile from a “master” surface into formable material, such as polymer. Hot embossing was used, for example, for the fabrication of Fresnel microlenses and lenslet arrays. Injection molding was used for the fabrication of micro optical lenses and for compact discs, and casting—for spectroscopic gratings. These methods can be used for replicating any surface relief microstructure with very high resolution, at an extremely low cost. Such elements are available, for example, from Digital Optics Corporation of Charlotte, N.C., and from Lasiris of St. Laurent, Quebec, Canada.
Similar molding or replication techniques have not yet been applied for optical materials transmitting in the mid IR. In metallurgy there are four basic metal forming processes: casting, machining, consolidating of smaller pieces and deformation. Within the metal deformation processes, cold working includes the processes that change the shape and form of the metal, when the metal is at a temperature below its softening temperature. The cold working processes include press forming or stamping, rolling, extrusion, drawing and forging. In some of these, which we may define as “bulk” forming, a metal ingot is pressed between two opposing dies, or between a punch and a die, and its bulk shape is changed. Forging, for example, can be carried out in open dies or in closed dies (also called closed impression dies.) The closed die can be made of several components, and it is then called segmented die or split die. In this case the segments can be separated from the forged element at the end of the forging process. This is very similar to the split cavity mold used for plastics, as mentioned above. In other “bulk” processes the metal is squeezed between rollers or extruded through an open die to form wires, tubes etc. There are cold work processes that may be defined as “surface” forming. For example in a stamping process, the stamp has a raised pattern cut on it and when it is pressed against a metal blank it generates a design on the surface of the blank. All these cold working processes are less expensive, the sizes of the parts are more accurate and the surface finish is much better than in hot processes.
Cold working processes cause, in general, hardening of the prepared bodies, which is frequently advantageous in metallurgy. If the extra hardening is undesirable, annealing at elevated temperatures can soften the bodies. Some cold working processes, such as extruding and rolling, are carried out in multiple steps. It often is desirable to soften the work piece between steps by annealing the work piece.
It would be highly desirable to be able to use some of these well-established methods for the shaping of blanks of infrared transmitting materials into optical elements, such as lenses or windows, or modifying the surface of infrared elements to form diffractive optical elements or arrays of refractive microlenses. It is bound to reduce the time and the cost of making these elements. This would be of particular interest in the case of optically transparent crystalline materials whose handling is difficult and time consuming (especially for large elements). Unfortunately almost all of these crystals are hard and brittle and much less ductile than metals and it is difficult to deform them without fracture.
During the last thirty years there have been attempts to apply pressure to single crystals and press forge or extrude infrared optical elements. These elements are polycrystalline and after pressing they should have the same density and the same optical characteristics as the starting material. Hot press forging was applied mostly to alkali metal halides and alkaline earth metal halides, which are frangible and tend to cleave. Both uniaxial and hydrostatic press forging were tried. The temperature used in these experiments was relatively high, on the order of 500° C. This temperature was lower than the melting point Tmp but higher than ½ Tmp (U.S. Pat. Nos. 3,933,970; 4,089,937; 4,171,400 and 4,522,865). Similar crystals have also been extruded to form desirable shapes (U.S. Pat. No. 4,839,090). There have also been attempts to apply high temperatures and pressures to compress powders of the same materials and form infrared elements (U.S. Pat. Nos. 4,013,796, 5,643,505 and 5,658,504). Finally, infrared transmitting chalcogenide glasses were prepared by molding at temperatures higher than the melting point of the glass (U.S. Pat. No. 5,346,523). Apparently there are technical hurdles that have not yet been surmounted. Inexpensive, mass produced, infrared elements made of any of the materials mentioned above are not available in large quantities.
There are very few infrared transmitting crystalline materials that are relatively soft and ductile. The hardness of materials can be conveniently measured by the Knoop method. For ductility, one may apply stress on a sample of length A and measure the length B when the sample fractures. The value (B−A)/A, the elongation ratio, is a measure of the ductility. The elongation ratio of metals is often as high as 50%. A small number of crystalline halides such as thallium halides (e.g. TlCIBr) or silver halides (e.g. AgClBr) have Knoop Hardness values of 10-20 and their elongation ratio is similar to that of metals unlike that of brittle crystals such as alkali halides. The mechanical and the photographic properties of silver halides have been studied for more than fifty years. Silver halides “flow under pressure” and have been called: “transparent metals.” The studies included mechanical deformation, yield stress measurements and research on strain ageing. See, for example, M. T. Sprackling and H. Shalitt, “Strain ageing in silver chloride: II The effect of temperature on the representative curve”, Philosophical Magazine, Vol. 48, pp. 383394, 1984. Almost all these works reported measurements on single crystals of AgCl and were carried out under tension.
Cold working methods have already applied to form three types of element from halide crystals:
I. Rolled Sheets: Some companies (e.g. the former Crystal Division of Harshaw Chemical Co., Solon Ohio, now Bicron Crystal Products of Washougal WA) have used hot—rolling techniques to prepare sheets of AgCl and AgBr of thickness of a few mm. These sheets contained various impurities with a total concentration of 40-50 ppm (see for example L. C. Towle, “Shear Strength of Silver Chloride”, J. Applied Physics, Vol. 37, pp. 4475-6, 1966) and they were photosensitive.II. Extruded Optical Fibers: IR transmitting optical fibers have been fabricated from silver halide or thallium halide crystals by an extrusion through an orifice of diameter 0.5-0.9 mm at temperatures between room temperature and 200° C. There are more than fifty patents discussing different fiber structures or extrusion techniques (see for example U.S. Pat. Nos. 4,188,089, 4,253,731, 4,315,667, 4,504,298, 4,721,360, 4,828,354, 4,865,418, 4,955,689, 5,575,960 and 5,602,947). IR transmitting fibers are manufactured by Oxford Electronics Ltd., Oxfordshire, UK, by CeramOptec GmbH, Bonn, Germany and by Art Photonics, Berlin, Germany. The optical transmission of these fibers is still not satisfactory, and they are not widely used.III. Press Forged Lenses: There are more than ten Japanese patent applications that were published between 1984 and 1994 and which describe the pressing of silver halide or thallium halide crystals between two lens shaped dies, to form lenses. The forging is done at pressures of 5-10 Ton/cm2 and at temperatures 200-300° C. (Japanese published patent applications: 59-212801; 04-170501; 04-340501; 04340502; 05-139764; 06-011604). None of these lenses is available commercially, and they have not been discussed in the scientific literature.
Summarizing this prior art, one may conclude that the three methods mentioned above cannot be used for mass production of infrared transmitting optical elements of high quality. The rolling technique described in (I) can be used only for making thin flat sheets, and the optical quality of the sheets obtained was rather poor. The extrusion of crystals through open dies described in (II) is a unique method for making fibers, and it is totally unrelated to the fabrication of optical elements such as windows or lenses. The press forging process mentioned in (III) describes a method for making lenses. In extensive work done by us, we discovered great disadvantages of this forging method. When one tries to forge elements at relatively low temperature (below 120° C.) the lenses crack. When one tries to forge elements at relatively high temperature (above 180° C.) there appears a chemical interaction between the crystals and the walls of the pressure chamber. This causes two problems: (i) the optical elements darken and their transmission is very poor. (ii) the walls of the chamber gradually deteriorate and become useless. It is possible to protect the top and bottom pressure plates by a thin layer of carbon or non-reactive metal, but it is not possible to protect the sidewalls. It is not practical to replace the very expensive pressure chamber after every forging procedure. This is probably why infrared lenses are not made today by forging.
In addition, there were several problems that interfered with previous studies:
(1) almost all these studies referred to single crystals or polycrystalline samples of AgCl or AgBr and not to alloys, such as AgClBr; (2) in most cases, the samples contained impurities in concentrations higher than 10 ppm; (3) most of the samples were photosensitive: they darkened under UV or visible light. All these ductile halide crystals are potentially suited for the fabrication of a plethora of infrared elements by the well-established metallurgical cold working methods. They have not been widely used, only because of the severe problems mentioned above.
The present invention addresses two issues: (a) the fabrication of infrared optical elements by metallurgical methods, using a novel method based on sacrificial dies. (b) The use of these elements in the various infrared systems, for infrared detection, thermal imaging, power transmission and infrared spectroscopy. Particularly in the case of silver halides, the present invention focuses on four major points: (I) the use of alloys of AgClxBr1-x with mechanical properties that are superior to those of AgCl or AgBr; and (II) the use of ultra pure materials, with total concentration of impurities lower than 10 ppm, to prevent the problem of darkening of the samples. (III) The use of a sacrificial split die, instead of using a simple pressure chamber. The segments of the die are separated from the optical element at the end of the process and they are discarded. (IV) Forge pressing at a well defined temperature range 120-180° C. As mentioned, forge pressing at lower temperatures gives rise to cracks in the optical elements (especially in the case of large elements), and forge pressing at higher temperature gives rise to darkening.
Attenuated total reflection (ATR) spectroscopy, also known as evanescent wave spectroscopy (EWS), has been used during the last few decades for the study of pastes, liquids and powders, in which intense scattering or absorption precludes the transmission of a probe beam. When light is totally internally reflected in prisms or waveguides, which are placed in vacuum, there is an evanescent wave, which decays exponentially outside the waveguide over a distance of a few wavelengths. In vacuum, this is not accompanied by any energy absorption. On the other hand, if instead of vacuum, the waveguide is immersed in a liquid or in another substance, the evanescent wave may be partially or totally absorbed and the transmission through the waveguide is reduced. This will predominantly occur at those wavelengths that correspond to the absorption spectrum of the sample. The spectrum of the transmission losses is the basis of ATR spectroscopy and the prism or waveguide in contact with the sample are called ATR elements.
A system for ATR spectroscopy includes three basic parts: a tunable IR source, an IR detector and an ATR element. In the MIR spectral range 3-30 μm, the commonly used light sources are a heated black body with a variable filter or a tunable diode laser. Some of the infrared detectors operate at room temperature (e.g. pyroelectric detectors) and others—at 77° K (e.g. HgCdTe detectors). The black body IR source and the IR detector may also be part of a Fourier Transform Infrared (FTIR) spectrometer. A standard ATR spectroscopy system is schematically shown in FIG. 1. The standard “bulk” ATR element is a thick waveguide with two beveled ends. IR radiation from a tunable IR source is totally internally reflected inside the waveguide. The radiation propagates until it reaches an IR detector. The sample may be in contact with one or two broader surfaces of the waveguide.
The standard ATR elements are made of materials that are highly transparent in the MIR, and they include single crystals of ZnSe, ZnS, Ge and diamond. There are some basic shapes of the elements that have been commonly used and that may be called “bulk” ATR elements. Some of these elements are thick (>3 mm) waveguides, in which there are multiple internal reflections. These include flat crystals with beveled ends, or cylindrical rods with pointed ends. There are other “bulk” ATR elements that are not waveguides, but shaped like pyramids, pointed prisms, spherical surfaces etc. In many cases it is required to couple the light into the elements using a special arrangement of flat or of spherical mirrors. Several of the commonly used ATR elements are shown in FIG. 2. FIG. 2A shows a cross section of half a sphere (or half a cylinder) 10 whose flat surface 12 is contacted with a sample. FIG. 2B shows a cross section of a pyramid (or a prism) 14, whose flat surface 16 is contacted with a sample. FIG. 2C shows a cross section of a pyramid (or a prism) 18 whose pointed end 20 is contacted with a miniature sample area. FIG. 2D shows a diamond shaped ATR element 22. FIG. 2E shows a cross section of a thick waveguide (or a cylindrical rod) 24 with two pointed ends 26. FIG. 2F shows a cross section of a thick waveguide (or a cylindrical rod) 28 with one flat end 30 and one pointed end 32. Also shown in FIGS. 2A, 2B, 2C, 2E and 2F are illustrative ray paths 34.
ATR spectroscopy has been widely used for the study of IR spectra of samples, which are not easy to measure by conventional transmission techniques. It has been used, for example, for measuring solutes in solutions, for the analysis of drilling fluids and the deterioration of engine oil, for measuring kerosene in oil shale, for determining the energy content of hydrocarbon fuel, and for chemical analysis of materials. It has also been used for determining compositional changes in materials, for surface analysis of silicon wafers, for measuring the ingredients of beer or dairy products, for determining the concentration of various substances in blood on the skin, and for measuring minute amounts of organic pollutants in water. (U.S. Pat. Nos. 3,902,807, 4,321,465, 4,553,032, 5,049,742, 5,252,829, 5,362,445 and 5,452,083).
In most cases a broad surface of the “bulk” ATR element is in contact with the sample. But, there are cases where the tip of the pointed part of a cylindrical rod or a pyramid is placed in contact with a small area on a sample. This could be used for ATR spectroscopy of microscopic samples or areas.
The original “bulk” ATR elements mentioned above were based on large segments of single crystals, such as thick plates or rods or pyramids, that had been cut, ground and polished to a desired shape (U.S. Pat. Nos. 4,595,833, 4,730,882, 4,746,179, 4,988,195, 5,015,092, 5,035,504, 5,172,182, 5,200,609, 5,229,611, 5,434,411, 5,440,126, 5,459,316, and 5,703,366). These have the advantages of a large surface area contacting the sample, well established fabrication methods and ease of determining and maintaining a specific angle of incidence of illumination between the plane, parallel surfaces. However, the “bulk” elements are large, expensive and susceptible to damage, because of mechanical scratches and because of chemical interaction with the measured samples. Furthermore, these elements are not flexible and they require frequent polishing to maintain their useful qualities.
There is a need to find a way of producing “bulk” ATR elements using processes that are faster, more flexible and much cheaper. Such elements could be replaced frequently, if needed, and they will make ATR spectroscopy a more widely used method. This is a motivation to use cold working methods for the forming of ductile IR transmitting crystals. This can be applied for the preparation of “bulk” ATR elements (including thick waveguides).
With the development of new fibers that are extruded from crystals that transmit well into the mid-IR (out to about 25 μm), it was proposed by various workers to use these fibers both as IR cables and as fiberoptic ATR elements. The IR fibers can be long and flexible, so they can be bent to conform to the surface to be measured. They are also inexpensive to manufacture, and can be replaced after each measurement, if necessary. These features led to the development of Fiberoptic Evanescent Wave Spectroscopy (FEWS). In a FEWS system two long lengths of IR transmitting fibers coated with a thick plastic jacket serve as IR cables. The IR cables transmit the light from the IR source to an ATR element (bulk or fiberoptic) and from this sensing element to the IR detector. A short section of an unclad fiber may serve as an ATR element that is in contact with a sample. Such a system is shown schematically in FIG. 3 and was the topic of several patents (U.S. Pat. Nos. 4,798,954, 4,827,121, 5,525,800 and 5,585,634). FIG. 3 is a schematic illustration of a Fiberoptic Evanescent Wave Spectroscopy (FEWS) system based on two long IR cables and a sensor (ATR) element that can be a short segment of cylindrical unclad IR fiber or a thin flatwaveguide. There are cases where it is advantageous to replace the fiberoptic sensor element in FIG. 3 by a “bulk” element and this was the topic of other patents (U.S. Pat. Nos. 4,829,186, 5,170,056 and 5,440,126). The main advantage of both FEWS systems (with “bulk” or with fiberoptic sensing elements) is that they can be used to carry out spectroscopic measurements in remote locations.
It has also been suggested to use mixed silver halide IR fibers as the ATR elements. These crystalline fibers, of the general formula AgClxBr1-x, with 0<x<1, are described in S. Shalem et. al., “Mechanical and Optical Properties of Silver Halide Infrared Transmitting Fibers”, Fibers and Integrated Optics, vol.2 no.9, pp.872-879 (1996). Fibers of diameter 0.5-1 mm and lengths up to 10 meters can be fabricated. These fibers are transparent between 0.5 and 25 μm, with a transmission loss minimum of about 0.2 dB/meter at 10 μm. Such fibers are nontoxic, non-hygroscopic and flexible.
It has been found that Fourier Transform (FTIR) FEWS systems, using these silver halide fibers as waveguides, can be used to perform measurements on a variety of samples, such as thin layers, liquids, powders and gases at various pressures. Furthermore, FEWS systems based on tunable diode lasers (TDL) or quantum cascade lasers (QCL) can be used to detect pollutants in water, which is important for environmental protection, in general, and in particular, for monitoring pollutants such as hydrocarbons or pesticides in water. The fiberoptic sensing element may be coated with a suitable plastic coating that causes enhancement of the pollutant signal. With such a coating the system can detect a few parts-per-billion of pollutants Such as chlorobenzene in water in a remote location and in real time.
However, even these cylindrical fibers are not ideal as sensing elements. While they overcome the problems of difficulty of working, they are still problematic with regard to the angle of incidence. Optimum configurations must be found since the signal-to-noise ratio increases as the fiber diameter decreases, while if the diameter is very small, it is difficult to couple light into the fiber.
One attempt to solve this problem is by tapering a central portion of a cylindrical fiber. An example is U.S. Pat. No. 5,239,176 to Foster Miller. This patent discusses tapering the optical fibers and indicates that silver halides are also suitable materials for the sensor fiber core. Very small pieces of chalcogenide glass fiber (2.5 cm long and 400 microns in diameter) were tried, but did not have adequate sensitivity. It was then discovered that reducing the diameter of a chalcogenide fiber could increase its sensitivity. This can be done by heating the fiber and pulling it to create a taper. Other attempts involved stripping the central part of a core/clad fiber and leaving the unclad section as the sensor element or bending the fiber (U.S. Pat. Nos. 5,416,579, 5,436,454 and 5,525,800). Also, the fiber sensor element was coated with various plastic coatings, either to protect it or to enhance the obtained signals (U.S. Pat. No. 4,893,894). Some of these elements are schematically shown in FIG. 4. FIG. 4A shows a short section 36 of unclad fiber. FIG. 4B shows a section 38 of clad fiber, where the cladding 40 has been removed at the center part 42, exposing an unclad section 44. FIG. 4C shows a segment of unclad fiber 46 that had been tapered: the two ends 48 have larger diameters than the central section 50. FIG. 4D shows a bent section 52 of unclad fiber 54. FIG. 4E shows a segment of unclad fiber 56 coated with a very thin protective layer 58. FIG. 4F shows a section of unclad fiber 60 coated with a layer 62 of porous polymer. Solute molecules (e.g. pollutants in water) can diffuse in, but the solvent (e.g. water) cannot. This gives rise to “enrichment” and higher detectivity.
There are, however, a number of disadvantages in the use of these tapered fibers. First, due to their cylindrical cross-section, a unique angle of incidence cannot be maintained, so calibrations and calculations are necessary when they are used. Second, chalcogenide glasses can be difficult to work with, because of their brittleness and the small area of contact between the tapered (cylindrical) sensor and the sample.
There is thus a widely recognized need for, and it would be highly advantageous to have, a thin flat waveguide which is flexible like a fiber, but which has the optical advantages of a thick plate or prism. An important issue addressed by the present invention is how to make thin waveguide ATR elements and how to couple IR radiation into and out of these waveguides.
During the last few decades optical and spectroscopic methods have been developed rapidly in the visible spectral range. Modem optical methods, making use of lasers and optical fibers, holography, and modern imaging technologies have been used in many scientific and industrial applications for diagnosis of materials, analysis of reactions etc. There has been an attempt to use very similar methods in medicine. In particular it had been expected that such methods could be used in clinical chemistry (e.g. blood analysis) or in pathology (e.g. in early diagnosis of diseases) for measurements in situ and in real time. This has proven to be much more difficult than had been anticipated. There are still severe obstacles in using modern optical methods for tissue diagnosis or blood analysis.
During the last few decades there has also been a rapid development in the field of infrared science and technology. In particular, infrared spectroscopy has been improved and the combination of new infrared detectors, modern electronic equipment and powerful computers, led to a revolution in this field. Methods such as FTIR spectroscopy are being used now in many areas in science and technology. These include the ATR methods mentioned above (U.S. Pat. Nos. 4,169,676 and 5,452,715).
There have been attempts to use IR spectroscopy in the field of medicine. One of the areas tried is IR clinical chemistry. The advantage of IR in this field is that sample preparation is easy, there is no need to use chemical reagents, and the results are obtained in real time and in situ. Biological fluids such as blood or urine or the synovial or interstitial fluids have been analyzed by spectroscopic IR measurements (including ATR measurements). The very preliminary results indicate that blood analysis can be done by these techniques. A second area where IR spectroscopy could be very useful is IR pathology. Various tissues have been analyzed, such as diseased and healthy tissues in the case of Alzheimer disease, plaque and healthy artery walls in cardiology etc. One of the most important tissues to be studied is cancerous tissue, in an attempt to obtain early cancer detection. Several groups have found that the FTIR spectra of different types of cancerous tumors are different from those of healthy tissues (U.S. Pat. Nos. 5,038,039, 5,539,207 and 5,596,992). Still, IR spectroscopic methods, including FTIR—ATR spectroscopy of tissues and biological fluids, have not yet gained acceptance among physicians.
Simpler optical methods have been applied in the field of cosmetics. The interest there is in skin analyzers. In this field it is desirable to determine, for example, the water content or the fat content of the facial skin. The methods used rely on scattering of light or on determining the color of the skin, but, again, these methods are limited (U.S. Pat. Nos. 4,494,869, 5,094,248 and 5,745,217).
It is anticipated that the uses of ATR infrared spectroscopy, using cold pressed ATR elements or thin waveguides, as sensing elements, will change the situation. Such elements would be useful for tissue diagnosis and for blood analysis, in medicine. Unlike the standard, bulky ATR elements, the pressed ATR elements, and especially the thin flattened guides, could be used for measurements inside the body. They could be inserted into the body (for example under the skin) via hypodermic needles and used for single measurements or continuous measurements. They could also be inserted into the body via standard endoscopes. These pressed ATR elements would also be useful in cosmetics for skin analysis or for the measurement of the penetration of drugs or cosmetic lotions or ointments into the skin.